What are the advantages of using a schema-free database like MongoDB compared to a relational database?

Question

I'm used to using relational databases like MySQL or PostgreSQL, and combined with MVC frameworks such as Symfony, RoR or Django, and I think it works great.

But lately I've heard a lot about MongoDB which is a non-relational database, or, to quote the official definition,

a scalable, high-performance, open source, schema-free, document-oriented database.

I'm really interested in being on edge and want to be aware of all the options I'll have for a next project and choose the best technologies out there.

In which cases using MongoDB (or similar databases) is better than using a "classic" relational databases? And what are the advantages of MongoDB vs MySQL in general? Or at least, why is it so different?

If you have pointers to documentation and/or examples, it would be of great help too.

Answer 1

Here are some of the advantages of MongoDB for building web applications:

1. A document-based data model. The basic unit of storage is analogous to JSON, Python dictionaries, Ruby hashes, etc. This is a rich data structure capable of holding arrays and other documents. This means you can often represent in a single entity a construct that would require several tables to properly represent in a relational db. This is especially useful if your data is immutable.
2. Deep query-ability. MongoDB supports dynamic queries on documents using a document-based query language that's nearly as powerful as SQL.
3. No schema migrations. Since MongoDB is schema-free, your code defines your schema.
4. A clear path to horizontal scalability.

You'll need to read more about it and play with it to get a better idea. Here's an online demo:

http://try.mongodb.org/

Answer 2

There are numerous advantages.

For instance your database schema will be more scalable, you won't have to worry about migrations, the code will be more pleasant to write... For instance here's one of my model's code :

class Setting   include MongoMapper::Document    key :news_search, String, :required => true   key :is_availaible_for_iphone, :required => true, :default => false    belongs_to :movie end 

Adding a key is just adding a line of code !

There are also other advantages that will appear in the long run, like a better scallability and speed.

... But keep in mind that a non-relational database is not better than a relational one. If your database has a lot of relations and normalization, it might make little sense to use something like MongoDB. It's all about finding the right tool for the job.

For more things to read I'd recommend taking a look at "Why I think Mongo is to Databases what Rails was to Frameworks" or this post on the mongodb website. To get excited and if you speak french, take a look at this article explaining how to set up MongoDB from scratch.

Edit: I almost forgot to tell you about this railscast by Ryan. It's very interesting and makes you want to start right away!